This invention relates generally to motion detectors for producing an electrical signal whose value depends on the degree of displacement, and more particularly to a differential reluctance displacement transducer system which is highly stable and yields a signal that is linearly proportional to input motion.
For measuring small displacement in the order of 0.1 inches, use is often made of a linear or differential transformer in which a core associated with a single primary and a pair of secondaries is longitudinally displaced by the motion to be monitored. While such transformers are effective as motion detectors in metering small displacements, they are not sufficiently sensitive and stable to afford accurate measurement for minute displacements in the milli or microinch range.
Thus when a motion detector is required to measure the displacement of a force beam operating on the open-loop principle, the detector must be capable of sensing the slightest beam movement to produce a signal that is linearly related thereto. The 1976 patent to Kazahaya, U.S. Pat. No. 3,968,693, discloses an open-loop differential pressure transmitter that is adapted to monitor various process variables, such as flow rate or liquid level by means of a pivotally-mounted force beam. Applied to the lower end of the force beam is an input force causing the beam to deflect to an extent depending on the variable being sensed. Operatively coupled to the upper end of the beam is a motion detector serving to convert the beam deflection to a corresponding signal suitable for transmission to a remote station.
An acceptable motion detector for this purpose is the standard differential reluctance transducer, for this is highly sensitive to the slightest motion. A transducer of this type is constituted by a pair of identical transformers, each having a U-shaped core on one leg of which is wound the primary, and on the other the secondary. The transformers are symmetrically arranged with the poles of the cores of the transformers facing each other. An armature is disposed in the space between the poles.
The armature of a differential reluctance transducer is operatively coupled to an element whose displacement is to be measured, the armature being shifted laterally from its neutral position toward the poles of the first transformer and away from the other to an extent in accordance with input motion. As the armature is so shifted, the reluctance in the magnetic circuit in one of the transformers increases while a corresponding decrease takes place in the magnetic circuit in the other.
Applied to the primaries of both transformers is a high-frequency drive voltage, thereby inducing voltages in the secondaries whose amplitudes depend on the turns ratio and the coupling coefficient (coupling factor). The voltages yielded by the secondaries are equal only at the neutral position of the armature. By connecting the secondaries in series opposition, a zero output is developed at the neutral position, the differential output rising above zero as the armature is shifted from its neutral position. Any offset of the operating range from the neutral position is referred to as bias.
Under ideal stable conditions with a zero winding resistance, a constant drive voltage and a constant operating temperature, the output of a differential reluctance transducer is linearly related to displacement. But while it is known to provide regulation for the drive voltage, with existing types of such transducers under finite winding resistance and changing temperature conditions, the relationship between the input displacement and the output signal is unstable and non-linear.